The present invention relates generally to digital-to-analog converters and more particularly to a digital-to-analog converter performing an upconverter function.
Various types of wireless telecommunication systems use digital-to-analog converters (DAC). Such systems include satellite communication systems which use DACs both on-board and in terrestrial gateway stations. Other types of systems include cellular communication systems and the like.
DACs translate digital words into analog voltages or currents. In a conventional current switching DAC a digital word controls a bank of switched current sources. The output current, or voltage, is derived from the number of current sources switched to the output port. The switches control analog weighted current sources. Typically there is an assortment of binary and unary, or equally weighted current sources. Other current source weighting schemes are equally valid however. In the double upconverting wireless electronic communication system 10 shown in FIG. 1, a digital-to-analog converter 12 is followed by a low-pass filter 14 and an intermediate frequency upconverting mixer 16. A bandpass filter 18, an amplifier 20, a second upconverting mixer 22, and an output bandpass filter 24 is also included in the system 10. An RF upconverted signal is thus generated. In satellite payloads and perhaps even more critically in many hand-held wireless electronic communication devices, reduction of weight, size, and the number of components is important. Both mixers 16 and 22 have different clock inputs, also known as local oscillator (LO) inputs, denoted as LO1 and LO2. The mixer, filter, LO and amp circuitry consume power, size and add weight to the communication system.
There are three primary methods of upconverting a baseband DAC signal. The first method as depicted in FIG. 1 is to low pass filter a baseband DAC output, and then upconvert with one or more stages of analog mixer and bandpass filter. This first method is realized with three or more separate components, a baseband DAC, an analog filter(s) and analog mixer(s). While the analog mixer/analog filter approach is a reliable method, it generally is larger, more complex, costly, and requires more assembly than other approaches
The second method is to digitally mix the baseband signal prior to sending the digital data to the DAC. In such a system it is typically required that the DAC clock in the digital data at a rate greater than or equal to the mixer LO upconversion frequency to avoid frequency aliasing from another nyquist band. A substantial drawback of this approach is that the digital signal processing, for example the DAC decode logic, that follows the mixing function uses clock speeds far in excess of those necessary to support the data bandwidth, placing a heavy burden on the DAC digital receiving and decoding circuitry, especially for high mixer LO frequencies. The upconverting DAC described herein is far more efficient in its distribution of clock frequencies, insofar as most, if not all, digital circuitry of the DAC can be clocked at rates minimally sufficient to support the data bandwidth.
The third method is to pulse sample the DAC output with either a return-to-zero or return-to-mid level signal. One key advantage of the pulse sample approach over the second approach detailed above of digital mixing prior to DAC decoding is that the DAC input data rates and decoder circuit is clocked at the lower data bandwidth rates, not the upconverted rates. A drawback of the third method is that it does not peak the signal level in the desired upconverted nyquist band signal rendering it less power efficient. Further, the output signal amplitude is very sensitive to fluctuations in the sampling pulse width. Additional circuitry may be required to stabilize, increase the amount of filtering, and/or amplify the output signal in the desired upconverted frequency band.
It would therefore be desirable to provide a circuit and method for accurately converting a digital signal to an analog signal that reduces power consumption, size, weight and reduces the number of components therein.
The Digital-to-Analog Upconverter (DAU) described herein extends the function of a conventional DAC by incorporating a digital mixer into the output current switching array.
In one aspect of the invention, the digital signal is provided to the digital-to-analog upconverter such as through digital signal processing circuitry. A sample rate signal and a divided sample rate signal are provided by divider chain using the clock input. The divided sample rate signal is used to drive the decode circuit, which in many cases is a binary-to-thermometer decoder. The digital outputs of the decoder are combined with the sample rate signal in a digital mixer to create a digital control word. The digital control word switches a plurality of analog output switches to analog output port.
In a further aspect of the invention, a method for operating a digital-to-analog upconverter comprises generating a sample rate signal, dividing the sample rate signal by an integer to form a divided sample rate signal, converting a digital signal to digital decoded signal in response to the divided sample rate signal, digitally mixing the sample rate signal and the decoded signal, and generating an analog output in response to the digital mixer output signal.
Several advantages are achieved by (1) embedding the digital mixer into the current switches, and (2) mixing at an integer multiple of the DAC sample rate. First, as shown in FIG. 1, the low pass filter that is typically situated between the DAC and the first upconverter mixer is eliminated. In addition, FIG. 1 indicates that a second upconverting stage, bandpass filter, amp, and mixer may also be eliminated depending on the user""s application. Secondly, as FIG. 4 indicates, the upconverted nyquist band looks much like the first nyquist band of a baseband DAC in terms of preserving harmonic frequency spacing and relative power. A third advantage, also seen in FIG. 4D, is that the output power is peaked for maximum power efficiency in the upconverted nyquist upper and/or lower sidebands. A fourth advantage is that the DAU data rates and/or clock frequencies are typically lower or as low as other upconverting DAC approaches which translates generally speaking into lower power dissipation, lower I/O data rates, and greater ease of implementation. Finally, perhaps the greatest feature of the DAU is its simplicity. It vastly extends the usefulness and range of system applications of a generic electronic component, digital-to-analog converters, with a quite simple circuit modification.